DIY 2m/70cm yagi

It's around five years since I hosted a blog, and for the life of me I don't know why I want to do it again. Here goes anyway, and I'll kick off with a post already prepared which I put up on Scunthorpe Steel Amateur Radio Club web site. It's a great site over there. Pay them a visit and see for yourself. http://www.g4fuh.co.uk/

Having enjoyed my first foray in the RSGB March 2m contest and keen to include 70cm SSB in my repertoire, I decided to replace my ageing 2m 2 element HB9CV antenna with a higher performance home brew dual band 2m/70cm yagi.

I spent several weeks trawling the internet for yagi plans, and finally decided on one of the antenna designs describe on Martin Steyers' excellent site ( www.qsl.net/dk7zb/ ). The antenna I chose was 5 elements on 2m and 8 elements on 70cm (see Fig.1), fed with a single coax, to be driven by two transceivers via a duplexer. Not the best option for DX maybe, but a good compromise between low profile for use in a suburban environment, and a significant step up in gain from my current 2 element beam. Martin Steyer claims 8.5-9 dBd for this design. There are, of course, quite a few other websites offering yagi plans, but many seem to fall into one of two camps, either precise designs using components difficult to source, or with plans which were a bit ambiguous with respect to certain design features. Martin Steyers' site was selected because he gives a clear and comprehensive set of instructions specifying the critical dimensions, i.e. element lengths/diameters/spacings and feeding methods whilst leaving the detailed construction method up to the home brewer.

Fig.1 Antenna dimensions

My main criteria for the design I chose was to have a robust construction capable of accurately achieving all the critical dimensions using the simple collection of tools I had available. A hand held drill, a workmate, a hacksaw, and some round files were all that were needed to fabricate all parts.

Space is limited in a blog post so I won't go into too much constructional detail here. Figs. 2 and 3 below are worth a thousand words and illustrate how the antenna was fabricated.

Fig.2 Element construction

Other than the positioning of the holes in the sides of the abs box, none of these parts require close dimensional control because critical dimensions are set during final antenna assembly when the perspex element brackets are carefully aligned and clamped in position using mole grips to the boom. It is then easy to drill through each assembly and bolt it securely in position.

Fig.3 Driven element mounted in abs box

The 2m driven element is in two halves with a small gap between them. To hold this element rigidly a tight fitting plastic rod (or in my case an empty biro tube) was inserted through the gap and two self tappers, which also serve as coax attachment points, lock everything in place. Finally, the abs box was fixed to the boom with stainless self tappers, and the coax feed, soldered to copper washers was attached.

Tuning, with hindsight, was fairly straightforward, although fairly stressful at times due to my mistake in positioning the 40mm dia. 4 turn RF choke feeder coil approximately 150mm from the dipole. The plans clearly show it needs to be as close to the feed point as possible. See Fig.4 (Thanks Derek for pointing this out, and for the other tuning suggestions you made).

Fig.4 Feeding detail (photograph taken from DK7ZB website)

After correcting the above mistake the antenna was finished by waterproofing the abs box entry holes with liquid rubber and mounted above a rotator on the roof. SWR's on both 2m and 70cm SSB calling frequencies were 1.10

Initial antenna performance is encouraging. Directivity, and by implication gain, is clearly a lot better than that of my old antenna. Only time will tell how durable this design is, but at least after a week on the roof nothing has fallen off yet!